Project Summary. Numerous clinical studies have shown that cerebral microinfarcts are likely contributors to vascular cognitive impairment and dementia (VCID). However, the mechanism by which these small, but prevalent lesions lead to brain-wide neural dysfunction remains unknown. Our central hypothesis is that microinfarct injury leads to neural impairments that extend well beyond the restricted lesion cores seen during histological and radiological examination. These remote effects, when accumulated, are a mechanism by which microinfarcts cause large- scale disruption of brain function and cognitive decline. The rationale of the proposed research is to use a mouse model where the timing and location of microinfarcts can be controlled in order to better understand how they cause brain dysfunction. We plan to examine: i) the spatial extent and chronicity of functional impairments induced by individual microinfarcts, ii) the cumulative effects of multiple microinfarcts, and iii) the cellular/molecular changes that underlie their remote effects. Our model uses state-of-the-art methods for controlled optical occlusion of targeted cortical penetrating arterioles, individually and in multiples, to precisely and non-invasively form small regions of ischemic injury that mimic human microinfarcts. The associated injury processes can then be studied in vivo over time using parallel high-resolution two-photon fluorescence calcium imaging and 7T MRI to reveal detailed aspects of brain pathophysiology that are potentially invisible to MRI or histopathology. We further use behavioral paradigms that are sensitive to microinfarcts to uncover their effects on sensory perception and cognitive function. Aim 1 of the project tests the hypothesis that cortical microinfarcts induce sustained neuronal deficits beyond their lesion core following their strategic induction within the mouse vibrissa sensory system. It further examines whether aberrant change in excitatory-inhibitory balance contributes to these deficits. Aim 2 of the project tests the hypothesis that the accumulation of multiple microinfarcts, spatially distributed throughout the cortices of both cerebral hemispheres, is sufficient to cause subcortical white matter degeneration (assessed in vivo with diffusion MRI tractography and ex vivo with histology) and impairment in cognitive tasks. This work will complement clinical research on VCID in several ways. First, it will provide detailed mechanistic information on how, and to what extent, microinfarcts impair remote brain tissues. Second, it will clarify what aspects of microinfarct injury are visible or invisible to MRI, the primary means to detect these lesions during life. Third, it will provide unique in vivo MRI-ex vivo histopathology comparisons to reveal the underlying biological processes that cause MRI signal change during gray and white matter injury. Fourth, it will establish a first-of-its-kind in vivo experimental platform to study mechanisms of microinfarct-induced pathology and to gauge the utility of new therapeutic agents.